Refactor `normalizeDefaultValue()`

[Tigrov]

Related PRs

• https://github.com/yiisoft/db-pgsql/pull/370
• https://github.com/yiisoft/db-mysql/pull/363
• https://github.com/yiisoft/db-sqlite/pull/326
• https://github.com/yiisoft/db-mssql/pull/328
• https://github.com/yiisoft/db-oracle/pull/289
• https://github.com/yiisoft/active-record/pull/390

Q	A
Is bugfix?	❌
New feature?	✔️/❌
Breaks BC?	❌
Fixed issues	#854

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.60%. Comparing base (e29b097) to head (e141010).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #898 +/- ## ========================================= Coverage 99.60% 99.60% - Complexity 1525 1538 +13 ========================================= Files 81 81 Lines 3760 3789 +29 ========================================= + Hits 3745 3774 +29 Misses 15 15 ```

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